Immunoassay techniques in the past have often required a complete schedule of analytical steps for species separation and identification. More recently, however, techniques have been developed to obviate the need for bulk separation. These latter techniques rely on "in-situ" separation at the surface of a sensitized detector. Thus, for example, it is already known to detect the presence of antigens in blood samples by causing these antigens to be attracted into an adsorbing layer of a substance containing the appropriate antibody species, with this layer lying adjacent to the gate of an insulated-gate fieldeffect transistor (IGFET)--a so called Chem-FET. The current flow between the source and the drain of this transistor is modified in the presence of antigens in a test sample to which the gate layer is exposed. The transistor current is thus continuously monitored to detect the presence of antigens. After the test, the transistor is disposed of.
There are however, limitations in the response of such Chem-FET devices, and the high cost of such disposable devices mitigates against widespread adoption.
More recently, sensitive, and possibly lower cost, optical assay techniques have been proposed. See for example:--Proceedings of 2nd Optical Fibre Conference (Stuttgart 1984) page 75; "Detection of Antibody--Antigen Reactions at a Glass-Liquid Interface as a Novel Optical Immunoassay Concept", R.M. Sutherland, et al, of the Biomedical Group, Battelle, Geneva, Switzerland (1984). In accordance with the technique that they described, the complimentary antibody is covalently immobilized onto the surface of a planar or fibre optical waveguide. The reaction of immobilized antibody with antigen in sample solution is detected using the evanescent was component of a light beam which has been totally internally reflected many times within the waveguide. This evanescent wave has a characteristic penetration depth of a fraction of a wavelength into the aqueous phase, thus optically interacting with substances bound to or very close to the interface and only minimally with the bulk solution. The efficiency of this technique thus depends on tight confinement of the evanescent wave relative to the interface and this in turn requires a large differential in the value of refractive index on each side of the interface.